The present invention relates to a method of fabricating a spiral inductor required for embodiment of RF integrated circuits. More particularly, the present invention relates to a method of fabricating an inductor capable of improving a quality factor and decreasing a series resistance by using as a material of the inductor silver smaller in a specific resistance than aluminum used conventionally.
Passive elements such as inductors, capacitors, resistors and the like are necessary for construction of integrated circuits, ICs. The passive elements are separately mounted on a circuit board or are integrated on a semiconductor substrate by batch processes.
The latter methods have an advantage that a size of the integrated circuit can be greatly reduced, and one of the latter methods is illustrated in FIG. 1, in which an inductor is fabricated by forming a spiral metal interconnection 2 on a semiconductor substrate.
In such a conventional method of forming the spiral inductor, as shown in FIG. 2, a multi-layer structure is formed such that an insulating layer 4 is formed on a semiconductor substrate 3 and a first metal interconnection of aluminum layer 5 is formed thereon.
Next, the aluminum layer is patterned, an insulating layer 6 is formed thereon, the insulating layer 6 is patterned to form a via hole, and then the via hole is plugged 7.
Next, a second metal interconnection of aluminum layer 8 is formed on the resultant structure, the aluminum layer is patterned and an insulating layer 9 is formed on the whole surface, thereby fabricating the spiral inductor.
In order to improve the adhesion characteristics of the metal layer or in order to prevent the metal from being diffused into the semiconductor substrate and the insulating layer, titanium Ti and titanium nitride TiN or titanium tungsten TiW layers may be formed, before or after forming the metal layer.
Because the quality factor Q of an inductor is in inverse proportion to series resistance of the metal line, the spiral inductor made of aluminum could not provide a good quality factor and thus, there is a problem that such spiral inductor is not suitable for the integrated circuit operating at high frequency.
On the other hand, it is known that the inductor made of silver having a lower resistance than aluminum is capable of having the decreased series resistance of the inductor itself. However, it is difficult to fabricate a fine spiral metal line using silver, so that an inductor made of silver could not have been embodied up to now.
Therefore, the present invention is made in order to solve the aforementioned problems.
An object of the present invention is to provide a method of fabricating an inductor suitable for integrated circuits operating at high frequency, using silver in place of the conventional aluminum and capable of decreasing a series resistance and improving a quality factor thereof.
The above object can be accomplished by a method of fabricating an inductor using silver according to the present invention. The method includes the following steps. A first step is of forming a first metal layer on a first insulating layer, patterning said first metal layer, and forming a second insulating layer on the resultant structure. A second step is of patterning said second insulating layer to form a via hole and forming a plug in said via hole. A third step is of forming a third insulating layer on the resultant structure and patterning said third insulating layer to form a spiral groove. A fourth step is of forming a second metal layer in said spiral groove to form an inductor. And a fifth step is of forming a fourth insulating layer for protecting said inductor from a mechanical force or materials causing a chemical reaction.
It is preferable that said fourth step includes a step of successively forming a diffusion barrier layer for preventing said second metal layer from being diffused and a seed layer for facilitating formation of said second metal layer in said spiral groove, before formation of said second metal layer, thereby a multi-layer of said diffusion barrier layer, said seed layer and said second metal layer constitutes a metal line of said inductor.
Also, it is preferable that said diffusion barrier layer includes Ti/TiN alloy or Ti/TiW alloy.
Also, it is preferable that said seed layer includes a silver (Ag) or a palladium (Pd).
Preferably, said first metal layer includes an aluminum layer, said plug includes an aluminum or a tungsten, and said second metal layer includes a silver layer or a silver alloy layer.
More preferably, said second metal layer is formed by a sputtering or an electroplating method.
Most preferably, the method further includes a step of reflowing said second metal layer by heat treatment.
It is still more preferable that a temperature of said heat treatment is within a range of 300xcx9c500 centigrade and the heat treatment is performed in an ambient of oxygen or halogen gas. Also, it is still more preferable that the method further includes a step of heat treating said second metal layer in an ambient of hydrogen gas to remove said oxygen or halogen gas existing in said second metal layer.
Also, according to the present invention, an inductor is provided. The inductor includes the following elements: a semiconductor substrate; a first insulating layer formed on said semiconductor substrate; a first metal layer formed on a predetermined region of said first insulating layer; a second insulating layer formed on said first metal layer and said first insulating layer; wherein said second insulating layer has a via hole to expose said first metal layer; a plug layer formed in said via hole; a third insulating layer formed on said second insulating layer and said plug layer, wherein said third insulating layer has a spiral groove; a second metal layer formed in said spiral groove, wherein said second metal layer includes a silver layer or a silver layer alloy layer; and a fourth insulating layer formed on said second metal layer and said third insulating layer.
Preferably, the inductor further comprises a diffusion barrier layer and a seed layer formed between said third insulating layer and said second metal layer in said spiral groove.
According to the aforementioned present invention, because silver smaller in a specific resistance than the conventional aluminum can be used as a material of an inductor, a quality factor of the spiral inductor can be improved and a series resistance of the spiral inductor can be greatly decreased. Therefore, a spiral inductor according to the present invention is suitable for the integrated circuit operating at high frequency.